Use the name that matches no library directive (#158)

Fixes #156

The readme, and likely other documentation around the web, has links to
the dartdoc site using the library name "async", which is what the
library would be named in the docs if we omitted the `library` directive
entirely. It is easier to restore the old paths by naming the
library identically than to update all the links.
diff --git a/lib/async.dart b/lib/async.dart
index e1faaf1..a97fc65 100644
--- a/lib/async.dart
+++ b/lib/async.dart
@@ -5,7 +5,7 @@
 /// Utilities that expand on the asynchronous features of the `dart:async` library.
 /// {@youtube 560 315}
-library pkg.async;
+library async;
 export 'src/async_cache.dart';
 export 'src/async_memoizer.dart';